This invention relates generally to an inverter driving scheme for a lamp ballast, and more particularly to an integrated circuit inverter control device having a pin which performs several functions.
A conventional electronic ballast includes both an input stage and an output stage. The input stage provides a D.C. source of power for the output stage through conversion of an A.C. signal, obtained from a power line, to a D.C. signal. The output stage, which can be of the half bridge inverter type, drives a lamp. Control circuitry, such as disclosed within U.S. Pat. No. 4,952,849, is required to control the heating of the lamp filaments for conditioning the latter prior to ignition (preheating). Control circuitry should also control cut back in the power consumed by the filaments once the lamp ignites. Until the filaments have been sufficiently heated, the voltage applied to the lamp should be maintained at a relatively low level to avoid damage to the lamp.
Accommodating these requirements can be met through the use of separate power supplies. Individual power supplies, however, are undesirable from both a cost and space requirement.
It is therefore desirable to provide an improved inverter controller for controlling the different heating requirements of the lamp filaments during and after preheat without employing separate power supplies. The improved inverter controller should also control application of different lamp voltages required during and after preheat.